With the wireless communication industry continually expanding, there is more and more demand for antenna solutions which provide a combination of high performance, low cost and small size to support the increasing number of wireless protocols. As multiple antennas are integrated into portable wireless handsets to provide wide ranging functionality (including Bluetooth, WiFi, GPS, UWB etc.), size in particular has become a critical factor.
The Federal Communications Commission (FCC) has approved the operation of UWB systems in the 3.2-10.6 GHz band. The UWB system defines a means for short-range high data-rate wireless transmission between electronic devices using a stream of very narrow or short duration RF pulses. The short pulses produce a UWB data stream which occupies a wide band in the RF spectrum. However, the radiated power level of a UWB data stream is lower than the sensitivity of most narrow band electronic devices; hence, UWB devices do not interfere with other electronic devices operating over a narrow band even though the operating band may be inside the frequency range of the UWB data stream.
UWB systems are best suited to short-range, indoor applications such as Wireless Personal Area Networks (WPANs) in homes and offices. Since UWB has a far greater bandwidth than existing technologies, such as bluetooth and 802.11, high data-rate UWB has the potential to allow a whole new level of wireless connectivity. It enables the efficient transfer of data from digital imaging devices, wireless connection of printers and other peripherals to personal computers, and the high-speed transfer of files between portable devices such as wireless handsets & MP3 players It also allows the wireless connection of DVD players, BluRay™ players etc. to TV sets. Thus, a wireless home or office becomes a reality, where the cable clutter and lack of mobility that is traditionally associated with the connection together of numerous electronic devices is eliminated.
The wide operating band of a UWB device produces a number of design challenges for the electronics engineer. One such challenge is in the design of a suitable antenna. A typical UWB antenna is required to provide a similar performance level to a narrow band antenna except the performance must be maintained over a much wider frequency range.
For example, when integrated in a portable wireless handset, an antenna will typically have ground planes located near the active radiating elements. Such closely located ground planes cause the fields around the antenna to be pulled in towards the ground plane. The effect of bringing a ground plane near the active radiating elements of an antenna is to greatly reduce the band width of the antenna.
One approach to provide a broadband antenna suitable for UWB devices is taught in United States Patent US005828340A “Wideband Sub-wavelength Antenna”, J. Michael Johnson. The antenna taught by Johnson is shown in FIG. 1 and comprises a tapered monopole patch radiating element 10 which is printed on a dielectric substrate 4 and which extends from a ground plane 14 located adjacent to the feed point 18 of the antenna and provides good electrical characteristics over a wide operating band. However, the antenna taught by Johnson has the disadvantage of having a relatively large physical size and the further disadvantage that any ground plane brought close to either side of the antenna will cause deterioration in performance. One way to reduce the size of the antenna is to fold it back in on itself as taught in European Patent application EP1986270A1 “Antenna Device and Communication Apparatus Employing Same”, Kuramoto, which teaches a similar antenna to that of FIG. 1 except where the radiating element is folded so that the open circuit end is in line with the feed point of the antenna. Folding the antenna as taught by Kuramoto reduces the overall size of the antenna.